strength has 21 distinct synonyms spread across 7 of its 9 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is weakness.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the property of being physically or mentally strong
“fatigue sapped his strength”
capability in terms of personnel and materiel that affect the capacity to fight a war
“we faced an army of great strength”
the power to induce the taking of a course of action or the embracing of a point of view by means of argument or entreaty
“the strength of his argument settled the matter”
the amount of energy transmitted (as by acoustic or electromagnetic radiation)
“he adjusted the intensity of the sound”
capacity to produce strong physiological or chemical effects
“the toxin's potency”
the condition of financial success
“the strength of the company's stock in recent weeks”
permanence by virtue of the power to resist stress or force
“they advertised the durability of their products”
Synonyms, antonyms and the broader and narrower relations for strength come from Princeton University’s WordNet. Full sources and licences.